Fluid permeable membranes fabricated from a wide variety of polymers have been used extensively to separate gases, such as oxygen, nitrogen, carbon dioxide, methane, hydrogen, and other gases from gas mixtures. The efficiency of the fluid separation process is determined by the properties of fluid mixture, the membrane material and its structure. Preferably, the gas separation membrane is highly selective, i.e. the membrane has a high separation factor, high gas permeability, and is resistant to chemicals and temperature variations, and is mechanically strong. However, membranes with high selectivity are generally characterized by low permeability, while membranes with high permeability generally possess unacceptably low separation factors.
A variety of methods for enhancing the selectivity of fluid permeable membranes by modifying the characteristics of the membrane have been described. For example, Kramer et al, U.S. Pat. No. 5,215,554 describe a method for enhancing selectivity by modifying the interstices or recesses of the membrane substantially throughout the thickness of the gas permeable membrane. U.S. Pat. Nos. 4,311,573, 4,589,964 and 4,968,532, disclose graft polymerization of a monomer on to a preformed, saturated polymer substrate which has been subjected to ozonization prior to grafting. See also U.S. Pat. Nos. 4,486,202; 4,575,385; 4,654,055 and 4,728,346. Halogenation treatment techniques have also been used to modify gas separation membranes. See U.S. Pat. No. 3,062,905 and 4,828,585.
Although several methods that improve selectivity of specific membranes have been described, including surface modification methods, such methods have several disadvantages--long exposure times, high chemical concentrations and high treatment temperatures, and possibly, degradation of the membrane. Thus there is a need for an improved and cost-effective gas separation membrane which possesses high gas permeability and a high separation factor. The current method provides a cost-effective gas separation membrane with a significant improvement in selectivity with a commercially acceptable loss of permeability.